1. Field of the Invention
This invention concerns a method of treating sewage, and more specifically concerns a process for reducing pathogens within the sewage.
2. General Discussion of the Background
The appropriate disposal of sewage has long been a problem, and this problem continues to grow as population densities increase. Typical sewage contains such things as raw human and animal wastes, garbage, related domestic wastes, and perhaps even toxic metals. A broad variety of pathogens, such as bacteria, fungi, viruses, parasites and protozoans, survive in this organic environment. Industry has long sought a method of disposing of the wastes and pathogenic organisms without infecting the environment, humans, domestic animals, or the food chain.
Since great volumes of sewage sludge are produced each year, it would be desirable to reuse the sludge. For example, the sludge could be reused as landfill, fertilizer, or erosion control material. Such reuses are dangerous, however, if the sludge contains significant amounts of pathogens.
The U.S. Environmental Protection Agency (EPA) has recognized the problems which disposal of pathogen infested waste can present. In response to these problems, the EPA has issued guidelines at 40 C.F.R. .sctn.257 concerning land disposal of sewage sludge which contains pathogens. The EPA regulations recognize three separate categories of sludge: unstabilized sludge, sludge exposed to a process to significantly reduce pathogens (PSRP), and sludge exposed to a process to further reduce pathogens (PFRP). "Unstabilized sludge" has not been exposed to any pathogen reducing process, is not suitable for land disposal, and can only be incinerated, buried or heat dried. Sludge which undergoes a PSRP, such as anaerobic digestion, heat treatment, lime stabilization, or air drying, can be disposed on land only if public access to the land is controlled for a period of from 12 to 18 months. Finally, sewage which has undergone a PFRP has no disease related restrictions on reuse. Unfortunately, previous PFRP methods have been expensive, highly energy intensive processes such as irradiation or thermal processing which are unsuitable for transforming the sewage sludge into a readily reusable end product.
Another drawback with these PFRP methods is that they fail to destroy some parasites, such as the ascarid. The ascarid is a type of helminth worm that is a common parasite in the intestines of humans and animals. Particularly susceptible to helminthiasis (intestinal infestation with helminths) are ruminants such as sheep, cattle, goats, pigs, horses, and mules. A wide variety of antihelminthic agents have been discovered, and they have varying degrees of efficacy.
Among the classes of materials which are known to be toxic to helminths such as ascarids (Ascaris suum, A. lumbricoibes) are the 2-substituted benzimidazoles of U.S. Pat. No. 3,325,356, phosphoramidates of U.S. Pat. No. 4,269,829, acetyl and carbalkoxythioureidobenzophenones of U.S. Pat. No. 4,310,537, and avermectin and milbemycin compounds of U.S. Pat. No. 4,547,491. Such compounds, however, are intended for therapeutic use in individual animals and are unsuitable for general addition to sewage sludge.
It has previously been found that ammonia can be toxic to ascarids. For example, Chefranova and his associates have disclosed in the Tropical Diseases Bulletin, Vol. 76, No. 3, Abstract 556, and Helminthological Abstract Series A, Vol. 45, No. 11 (1976), Abstract 5830, and Vol. 47, No. 3, Abstract 1272, that treatment of sewage with 3-4% ammonia by volume destroyed all viable ascarids present. Reducing the ammonia concentration to 2%, however, left many of the eggs viable.
Other Russian researchers have found that relatively high volumes of ammonia are toxic to ascarids. Part of the problem encountered by the Russian researchers, however, has been that ammonia evaporates and reduces the concentration of toxic ammonia present to combat ascarids.
In the United States, Reimers et al. have also investigated helminths in sludge. U.S. EPA Publication 600/S2/81/166 (Oct. 1981) Order No. PB 82-102 344 and EPA/600/S1/185/022 (Jan. 1986) Order No. PB 86-135 407/AS. The 1981 study, which is part of the work leading to the present application, concluded that conventional sludge treatment processes (e.g. mesophilic anaerobic or aerobic digestion) were not very effective in destroying parasite eggs, and ammonification studies of ascarids were inconclusive. The 1986 study found that when ammonium sulfate at a dosage of 50 mg ammonia per gram of sludge was added to sludge previously aerobically digested at 25.degree. C. for 10 days, there was little effect on the ascarid eggs during the first 5 days. After ten days, 62% of the eggs were inactivated. When the ammonia concentration was increased to 500 mg/g solids, complete or near complete inactivation was observed after 10 days. When ammonia gas was added to sludges previously aerobically digested at 25.degree. C. at detention times of 10, 20, or 30 days, a dosage of 1% ammonia was necessary to obtain effective inactivation of the ascarid eggs.
A serious problem with ammonification of sewage sludge is that a large enough amount of ammonia must be added to the sludge to kill ascardis within a reasonable period of time. It was previously thought that at least 2% ammonia by volume was required to effectively destory most viable ascarids in sludge within ten days. If the sludge wa not sealed in an airtight reaction vessel, however, additional amounts of ammonia were required to compensate for volatilization.
Another waste product treatment process is disclosed in U.S. Pat. No. 3,837,872 which was re-examined and subsequently issued as Re-examination Patent B13,837,872. The '872 process concerns treatment of sewage sludge and other wastes by chemical fixation and physical entrapment of pollutants. The waste is treated by mixing it with a setting agent and silicate, preferably in a sewage to setting agent to silicate ratio of about 10:1:0.5 by volume. The resulting product is a friable, clay-like mass having a polymer lattice that entraps and prevents migration of toxic materials such as heavy metals and some organics.
The present inventors have found that the process of the '872 patent can reduce viability of ascarid eggs in sewage sludge. It is believed that the highly alkaline environment of the chemical fixation process hydrolyzes nitrogen containing wastes in the sludge to evolve some ammonia to kill some ascarid eggs. The previously patented process, however, still leaves about 60% of the ascarid eggs viable. Waste containing such a high percentage of viable ascarid eggs fails to satisfy environmental regulations for substantial elimination of parasites from treated sludge which is to come into contact with humans or their food chain. For safety and in order to qualify as a PFRP, at least about 99.9% of viable parasites must be destroyed. The level of indicator pathogens in a spiked sample must be reduced by three logs. Such a test is designed to ensure that actual municipal sludge treated with the process will substantially eliminate all the parasites, and the '872 process would not qualify.
The '872 process is, however, otherwise effective in satisfying EPA requirements for a PFRP with respect to bacteria and viruses. The highly alkaline environment produced by the '872 process is toxic to bacteria and viruses, reducing their total coliform at least three logs such that less than 0.1% of them survive the treatment. The '872 process fails, however, to qualify as a PFRP since a substantial percentage of parasites remain viable.
It is accordingly an object of this invention to provide a method of treating sewage sludge which substantially completely eliminates viable ascarid eggs from sewage sludge.
Another object of the invention is to provide a method of treating sewage sludge such that the sludge will satisfy EPA regulations for a process for further reducing pathogens (PFRP), thereby rendering the treated sludge suitable for profitable reuse as landfill, landfill cover, fertilizer or erosion control material.
Yet another object of the invention is to provide such a method which produces a product that is not subject to reinfection.
Even yet another object is to provide a method of sewage treatment which is economical and results in a nontoxic product.
A still further object of the invention is to provide such a method in which relatively small amounts of ammonia are required as compared to the prior art.
Finally, it is an object of this invention to provide such a sewage treatment process which can be performed on a continuous basis to treat large volumes of sewage sludge.